Today GOG.com started their Summer Sale for this year, which is to last until June 22. 12:59 PM UTC. Every day at 1:00 PM UTC, new deals are added and stay available for the rest of the sale.
You can get System Shock 2 for free if you access the sale page via their GOG Galaxy client during the first 2 days of the sale.
There's also a system in place where you can earn "XP" by performing certain actions or buying games that unlocks more free games (hovering over a game banner in the main page seems to show how much "XP" you can get.
You can get more info on it here and here
Feel free to both ask for and give recommendations/advices on all games on sale! or just wait for the Steam one